The present invention relates to a sheet-fed offset rotary printing press with a convertible press mechanism which turns a sheet over when the sheet is to be gripping-changed from an upstream transport cylinder to a downstream transport cylinder in a sheet convey direction to perform double-sided printing.
In a sheet-fed offset rotary printing press with a convertible press mechanism of this type, a sheet which has been conveyed with its leading edge being gripped by the grippers of an upstream transport cylinder in the sheet convey direction is not gripping-changed, when the grippers of the upstream transport cylinder oppose a downstream transport cylinder, and passes by the downstream transport cylinder. After that, when the trailing edge of the sheet opposes the downstream transport cylinder, it is gripped by the grippers of the downstream transport cylinder. In this state, the downstream transport cylinder continues rotation, and the grippers of the upstream transport cylinder release the sheet, to turn the sheet over. At this time, if the sheet stays in tight contact with the surface of the upstream transport cylinder and does not separate from it, the sheet may be damaged, or a gripping-change error occurs. In order to prevent this, air is blown to the upstream transport cylinder so that the sheet is separated from the surface of the upstream transport cylinder.
A conventional sheet-fed offset rotary printing press with a convertible press mechanism comprises an air blowing means fixed inside a notch in a convertible cylinder, as shown in U.S. Pat. No. 5,413,040. When a sheet is to be gripping-changed from an impression cylinder to the convertible cylinder, air is blown to the gap between the outer surface of the impression cylinder and the sheet to be separated from the outer surface of the impression cylinder, so as to separate the sheet which is in tight contact with the outer surface of the impression cylinder.
In the conventional sheet-fed offset rotary printing press with the convertible press mechanism described above, the air blowing means is fixed in the notch of the convertible cylinder. As the convertible cylinder rotates, the air blowing means also rotates to change the air blowing direction. This shortens the blowing time of the air from the air blowing means toward the sheet, so the quantity of air blown to the sheet becomes insufficient. The sheet thus stays in tight contact with the impression cylinder and cannot be reliably separated from the outer surface of the impression cylinder. The air blowing means is integrally rotated with the convertible cylinder. Thus, a hose or the like to supply air from an air source to the air blowing means must be provided in the convertible cylinder. This complicates the structure.